Which best describes a tendon?
a flexible tissue that acts as a shock absorber between bones
a somewhat elastic tissue that connects bone to either bone or cartilage
a flexible and compressible tissue that provides joint stability
a tough and elastic tissue that connects muscle to bone to move joints

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.